What exactly does "placing a hold" on a seat mean?
My travel agent always tells me that she has a seat on the airplane "on hold" for 24 hours so I can decide to buy the fare or not. But can I place a hold myself on any seat? For free? And for how long?

The answer is... it depends!

It depends on the fare rules for your itinerary. It depends on what booking engine you use.

A fairly typical fare rule will say "payment and ticketing must take place within 24 hours of booking". This means that a seat can be held for 24 hours without payment - after the 24 hours you either let it go, or you pay for it.

The problem is that while this is something you can do with the airline over the phone, or that your travel agent can do via their CRS, it's something that isn't always programmed into web-based booking agents available to the general public. When I book a KLM flight on the KLM web site I have to pay (and commit myself) immediately. Booking the same flight via Expedia I can hold it until midnight the following day.

My Travel Agent was able to hold my itinerary/booking for 30 days before I paid her. How did she manage that? (This was with AA/All Nippon) my trip to Japan.

Either the fare rules allowed it on this particular itinerary... or she was using some trick such as cancelling and re-booking every day. There is always some risk in doing this as someone could snatch up the seats in between her releasing and rebooking them, but she would be able to see how many seats are left in the relevant fare class, and as long as there are plenty then the risk is fairly small.

Mine were held for like 24 days when I flew to Japan on AC/JAL (Japan Airlines). Just paid when they notified my seat was allocated to me off the waitlist. And no, my travel agent didn't have to rebook the ticket over and over.

Sanosuke!

If you were waitlisted it's a totally different situation. The seat wasn't being held for you, it was being requested. You didn't have a seat until the waitlist cleared.

(I used to work in airline reservations, BTW!)

NOT ALLOWED. It's called churning, and it will get an agent in trouble (almost) as fast as back-to-back or hidden-city ticketing....